Is there an idiom expressing the idea that "things never change"? By that I mean, there will always be war, bad people, etc. Is there an idiom that you can use to mean that? The closest phrase, idiom I could think of is "It's just like the old days", but it doesn't really mean that "there are some things that will never change."
The Collins dictionary has the phrase
Twas ever thus
Things never change
This actually needs an apostrophe because
'Twas ever thus
is short for
It was ever thus
Which means: things were always like this.
There are a number of related idioms. For example Liron Ilayev's suggestion:
set in stone: "The doctrines of the Church are set in stone", the cardinal lamented, "not subject to the shifting winds of public opinion"
"Set in stone" suggests something (such as a tradition, practice, pattern, law, or rule) that is difficult to change, even if people want it to change. If, however, you want to suggest the people themselves are reluctant to change:
hidebound: The older members of the club are hidebound and will not consider changing the group’s membership requirements.
mired in tradition: Many educational establishments are too mired in tradition, and incapable of admitting that students can be taught in any number of ways that don't involve sitting in lecture for hours.
Some aphorisms that mean exactly this:
The more things change, the more they stay the same
You can't teach an old dog new tricks